The present invention relates to a new and improved construction of a direct indicator device for the determination of the cardiac output flow rate-- also known as the cardiac output-- according to the thermodilution method wherein there are provided two temperature feelers or sensors connected to the blood circulation for resepectively determining an inlet temperature and dilution temperature of a liquid injected into the blood circulation, the temperature of which deviates from the temperature of the blood.
The principle of the thermodilution method is predicated upon the fact that a certain volume of a liquid is injected into the blood circulation, the temperature of which liquid or injectate deviates from the temperature of the blood. By measuring the course of the temperature of the resultant blood-injectate mixture as a function of time the blood throughflow can be derived according to the well known Stewart-Hamilton formula. For determining the cardiac output or cardiac output flow rate, that is to say, the speed of passage of the blood through the heart there is selected, for instance, as the locality of the injection of the liquid the right atrium (right auricle) and as the locality of the thermodilution measurement the pulmonary artery. There is injected, for instance, 10 ml cold liquid within a few seconds, which for an expected cardiac output of 10 to 20 l/min. constitutes a disturbance which is of no great significance. The pumping function of the heart brings about an intensive degree of commingling or admixing of the blood and the injected liquid. A cold loss at the vessel walls does indeed occur, but since however the temperature gradient does not penetrate too deeply into the surrounding tissues, within the measuring time there is again compensated for the most part the temperature differences owing to the inflowing blood, so that finally no cold is lost. Moreover, inasmuch as the thermal compensation process extends over a number of heart beats (approximately 10) the throughput speed which varies as a function of time can be replaced by an average or mean value, so that there is also reduced the influence of the fluctuations in the temperature of the blood. In particular, the temperature of the blood in the pulmonary artery fluctuates by approximately 0.05.degree. C in rhythm with the breathing, which is to be compared with a temperature change of about 0.3.degree. C caused by the injection of 10 ml of a liquid which is colder by 15.degree. C.
During the construction of a direct indicating device for determining the cardiac output according to the thermodilution method there must be taken into account certain other disturbing influences. Firstly, with changes in position of the patient there oftentimes occur considerable changes in the blood temperature, necessitating the elimination of the measurement result. Then, the recirculation of the blood, i.e. the multiple passage of the same blood particles at the measurement location during the measurement interval necessitates interrupting the measurement after a certain time. Of course--one one of the major advantages of the thermodilution method in contrast to other known methods-- resides in the fact that the influence of the recirculation is markedly reduced owing to the action of the body as a thermostat, since with the rather long residence time of the blood in the capillary region there occurs at that location an extensive temperature compensation and, on the other hand, it has been found that the so-called rapid recirculation via the coronary vessels with a normal circulation system does not lead to appreciable distrubances in the measurement. Finally, compensation processes occur between the temperature of the injected liquid and the temperature of the infeed at the catheter. Consequently, the temperature of the injected liquid should be directly determined at its entry into the blood circulation in order to ensure for a faultless or error-free result of the thermodilution measurement.
Catheters are known to the art which possess an arrangement of a liquid infeed and two temperature feelers which are suitable for use with the thermodilution method. As the temperature feelers there are usually employed thermistors or equivalent temperature-sensitive elements. In this way there can be determined at suitable locations of the blood circulation system a respective measurement value designated as the inlet temperature and dilution temperature respectively.
According to a state-of-the-art apparatus for the determination of the cardiac output according to the thermodilution method the thermistor measuring the dilution temperature generates through the agency of a bridge circuit and an amplifier a voltage in the form of a signal which is integrated as a function of time. In this way there can be determined the integral of the dilution temperature curve. With a considerable percentage of the measurements the dilution temperature does not return to the starting value, so that the integration must be interrupted at an empirically determined point in time and the course of the curve must be interpreted by the operator for the purpose of either accepting or rejecting the measurement result. A direct indication, comparison and operation by unskilled laboratory personnel is not possible with such prior art equipment.
According to a further prior art device the evaluation of the measurement result is carried out by a computer. This computer is programmed in such a manner that the point in time of interruption of the integration as well as the acceptance or rejection of the measurement result is determined by the computer itself from the course of the dilution temperature curve. A result is delivered which is to be read-off of a digital voltmeter and which is to be compared with a calibrated value which is separately generated and indicated at the same digital voltmeter. The calibrated value is produced in such a way that with temperature feelers which are connected at the device the measurement values corresponding to the temperature are changed by a predetermined or pre-known amount, thereby simulating pre-known temperature changes. This is brought about, for instance, by applying an additional voltage at the relevant bridge circuit of the thermistor. Thereafter there are manually undertaken at the device the necessary adjustments in order to bring into coincidence the indication with the reference value of a likewise pre-known cardiac output. It should be recognized that notwithstanding the improved indication and comparison nevertheless the equipment cannot be operated by unskilled laboratory personnel.